T-cell exhaustion is a state of T-cell dysfunction that arises during many chronic infections and cancer. It is defined by poor T-cell effector function, sustained expression of inhibitory receptors and a transcriptional state distinct from that of functional effector or memory T-cells. Exhaustion prevents optimal control of infection and tumors. (E John Wherry, Nature Immunology 12, 492-499 (2011)).
T-cell exhaustion is characterized by the stepwise and progressive loss of T-cell functions. Exhaustion is well-defined during chronic lymphocytic choriomeningitis virus infection and commonly develops under conditions of antigen-persistence, which occur following many chronic infections including hepatitis B virus, hepatitis C virus and human immunodeficiency virus infections, as well as during tumor metastasis. Exhaustion is not an uniformly disabled setting as a gradation of phenotypic and functional defects can manifest, and these cells are distinct from prototypic effector, memory and also anergic T cells. Exhausted T cells most commonly emerge during high-grade chronic infections, and the levels and duration of antigenic stimulation are critical determinants of the process. (Yi et al., Immunology April 2010; 129(4):474-481).
Circulating human tumor-specific CD8+ T cells may be cytotoxic and produce cytokines in vivo, indicating that self- and tumor-specific human CD8+ T cells can reach functional competence after potent immunotherapy such as vaccination with peptide, incomplete Freund's adjuvant (IFA), and CpG or after adoptive transfer. In contrast to peripheral blood, T-cells infiltrating tumor sites are often functionally deficient, with abnormally low cytokine production and upregulation of the inhibitory receptors PD-1, CTLA-4, and TIM-3. Functional deficiency is reversible, since T-cells isolated from melanoma tissue can restore IFN-γ production after short-term in vitro culture. However, it remains to be determined whether this functional impairment involves further molecular pathways, possibly resembling T-cell exhaustion or anergy as defined in animal models. (Baitsch et al., J Clin Invest. 2011; 121(6):2350-2360).
Programmed cell death 1 (PD-1), also called CD279, is a type I membrane protein encoded in humans by the PDCD1 gene. It has two ligands, PD-L1 and PD-L2. PD-L1, also called CD274 or B7 homolog 1 (B7-H1) is a 40 kDa type I transmembrane protein encoded in humans by the CD274 gene.
PD-1 is expressed on the surface of activated T cells, and PD-L1 is expressed on the surface of antigen presenting cells (APCs), such as dendritic cells and macrophages. PD-L1 is also overexpressed in several tumors, including breast, lung, bladder, head and neck, and other cancers. When PD-L1 or PD-L2 bind to PD-1, an inhibitory signal is transmitted into the T cell, which reduces cytokine production and suppresses T-cell proliferation.
The PD-1 pathway is a key immune-inhibitory mediator of T-cell exhaustion. PD-1 functions to limit the activity of already activated T cells in the periphery during the inflammatory response to infection in order to limit autoimmunity. Blockade of this pathway can lead to T-cell activation, expansion, and enhanced effector functions. As such, PD-1 negatively regulates T cell responses. PD-1 has been identified as a marker of exhausted T cells in chronic disease states, and blockade of PD-1:PD-L1 interactions has been shown to partially restore T cell function. (Sakuishi et al., JEM Vol. 207, Sep. 27, 2010, pp 2187-2194).
Methods and compositions for the treatment of persistent infections and cancer by inhibiting the PD-1 pathway are disclosed in WO 2006/133396. Human monoclonal antibodies to PD-L1 are described in WO 2007/005874, US2011/209230, U.S. Pat. No. 8,217,149 and WO 2014/055897.